1. What is ultraviolet light?
Ultraviolet light (UV), referred to as such because in frequency it lies just beyond the visible color violet, is electromagnetic radiation with wavelengths that range from 10 nm to 400 nm and energy levels ranging from 3eV to 124 eV, thus setting it between visible light and X-rays on the electromagnetic spectrum. UV radiation is divided into a number of subtypes, the most common being UVA, also known as long wave UV or black light, UVB, also known as medium wave UV, and UVC, also known as short wave UV or germicidal UV. The sun emits all subtypes of UV radiation, but much of it is blocked by the Earth’s ozone layer. UV radiation can cause many different types of chemical reactions, the most well known being the tanning or sunburning of the skin, and it also causes many materials to glow or flouresce. There are many commonplace practical applications of UV radiation in the modern developed world.
2. What is ultraviolet germicidal irradiation and how does it work?
Ultraviolet germicidal irradiation (UVGI) refers to the use of a certain wavelength of short-wave ultraviolet light (UVC) in the sterilization of water, air, or object surfaces. Its use has become popular in the disinfection of wastewater and drinking water on the large and small scale. Microorganisms, including bacteria, viruses, parasites, fungi, and spores, have not developed resistances to UVC radiation since the Earth’s atmosphere blocks most of it, thus exposing these harmful microorganisms to UVC radiation with a wavelength of 253.7 nm emitted by a germicidal lamp, produces thymine dimers, or molecular lesions, within the micro-organismal DNA thereby rendering them unable to grow or reproduce, effectively destroying them. In order for this process to be successful, the microorganism must be in direct contact with the UVC radiation. Therefore, if sterilizing water, it can not be turbid because the particles in the water causing it to be cloudy may block the UV rays, thus the water must first be filtered or treated with activated charcoal for the UV sterilizer to function properly.
3. What is a germicidal lamp and how does it work?
A germicidal lamp is a special type of light bulb which creates the needed specific wavelength of UV radiation in sterilizing systems. Germicidal lamps are normally low pressure mercury vapor lamps, which are gas discharge lamps that use mercury in an excited state to produce light. As electricity arcs through the mercury vapor, the mercury ionizes causing it to radiate ultraviolet light. They are made of a special titanium fused, or “doped”, quartz glass, meaning they contain primarily silica in amorphous (non-crystalline) form. This type of quartz must be used in order for the desired radiation frequencies to pass through the glass uninterrupted. When the bulb is not doped with titanium it also emits radiation at a frequency of 185.3 nm which creates ozone gas, which though harmful to human cardiopulmonary health if inhaled, has many practical applications including disinfection and deoderization.
4. What are the pros / cons of germicidal irradiation?
UVGI is an extremely effective form of disinfection, but it is dependant on a few factors. One such factor being the infective microorganisms’ line-of-sight exposure to to the UV light. The effectiveness of the lamp will be therefore lessened if the radiation is blocked by particles in murky water or dust or film coating the outside of the bulb. To increase effectiveness the light can be reflected, most commonly with aluminum. There are also potential dangers to human health that go along with exposure to UVC radiation. It causes sunburn in the skin and can, in extreme cases, cause skin cancer. It also causes painful inflammation of the cornea in the eye and can temporarily blind and even permanantly blind by damaging the retina. For these reasons, most UVGI systems have lamps that are shielded or are in environments that limit exposure. UVGI is now beginning to replace chlorination in many wastewater disinfection systems. Water chlorination creates toxic by-products and changes the chemical makeup of the water, effecting taste, while UVGI does not. On the other hand, chlorinated water is protected from reinfection, while water treated with UVGI can become reinfected. Thus, many times it is good to have the treatment as close to the end use as possible or the water must be transported in a way that will keep it from becoming contaminated.
5. What are the practical applications of UVGI?
Apart from sterilizing water, UVGI can be used in much the same way to disinfect air. UVGI air purifiers usually work by utilizing a fan to force air by a UV lamp shielded within a freestanding unit. Well placed UV lamps and a quality filtration system for removing dead microorganisms, are very effective ways of removing airborn pathogens. UVGI can also be used to sterilize hospital and laboratory apparati, including surgical tools, safety goggles, glassware, etc. It is also often used to disinfect biological safety cabinets (“hoods”). Pools, ponds, and aquariums can also be cleaned using UVGI. Circulating the water passed UVGI lamps in the filter can neutralize single-celled algae, thus maintaining water clarity and sterility. Since the FDA passed a rule requiring that fruits and vegetables meet HACCP requirements in 2001, UVGI has also been used to sterilize fresh juices including fresh-pressed apple cider. Ozone producing UVC lamps can be used on a large scale to decontaminate anything from a small closet to entire buildings with spore or mold problems. Of course these spaces must be cleaned and sealed to protect people from the harmful side effects of the ozone gas. Even hospital operating rooms are often bombarded with UV radiation after a surgery.
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